1. Field of the Invention
The present invention relates to an aqueous dispersion of polyester resin to be applied to a substrate to form a film with a high water-proofness and high solvent resistances.
2. Description of the Related Art
Polyester resin composed of polybasic acid components and polyhydric alcohol components and having a high molecular weight as resin for forming a coating film is excellent in processibilities of the coating film, resistances (solvent resistances) to organic solvents, weathering resistances, and adhesion properties to a variety of substrates, so that it is immensely used as a binder component in fields of coating materials, inks, adhesives, coating agents, and the like.
Especially, today the use of an organic solvent tends to be restricted from the standpoint of preserving environments, saving resources, restricting hazardous substances by the Fire Services Act or the like, and improving industrial hygiene and it has been thriving in development of an aqueous dispersion of polyester resin, in which polyester resin is finely dispersed in an aqueous medium as the polyester resin type binder to be employed for the above described purposes.
For example, Japanese Patent Laid-Open No. Hei9-296100 proposes an aqueous dispersion of polyester resin obtained by dispersing polyester resin with an acid value of 10 to 40 mgKOH/g and a weight average molecular weight of 9,000 or higher in an aqueous medium, and it is described that a coating film excellent in properties such as processibilities, a water proofness, and solvent resistances can be produced using such an aqueous dispersion.
The above described aqueous dispersion of polyester resin can be used as a coating agent for a variety of substrates and is capable of forming a high quality polyester resin coating film with a high adhesion strength, however if the dispersion is stored for a long duration, the molecular weight of the polyester resin tends to be lowered. Therefore, it is probable to cause a problem of deterioration of the properties such as the processibilities, the water proofness, the solvent resistances and the like of the coating film formed using the foregoing dispersion.
The present invention is to provide an aqueous dispersion of polyester resin being excellent in a stability on a molecular weight even if it is stored for a long time, its production method, and an aqueous coating composition.
The above object can be achieved by an aqueous dispersion of polyester resin having an acid value of 8 to 40 mg KOH/g and a weight average molecular weight of 9,000 or more, wherein the aqueous dispersion contains an organic solvent less than 0.5% by mass.
The above object can be achieved by a method for producing the aqueous dispersion of polyester resin described above, comprising at first a step of obtaining an aqueous dispersion of polyester resin containing not lower than 0.5% by mass of an organic solvent by adding the polyester resin and a basic compound to an aqueous medium to make the resulting mixture aqueous and then a step of removing the organic solvent from the aqueous dispersion.
The above object can be achieved by an aqueous coating composition being obtained by adding a curing agent to the aqueous dispersion of polyester resin described above.